By definition, a skewed distribution is primarily caused by a skewed data wherein the trend of the graph has a tendency to become distorted either to the left or right of the curve. In addition, among the main causes of it is when "<span>a sample that excludes subjects that are not part of the population being measured."</span>
if you don't have enough variability for every subject then some subjects are going to be left out which makes the data set skewed. (I got the question right but that was the explanation that helped me).
